Wondering what is a MOSFET and how it switches? It’s a transistor where voltage on the gate decides whether current flows between the source and the drain.
This page sits inside the wider topic of semiconductor basics, so start there if terms like doping are new to you.
| Item | Value |
|---|---|
| Terminals | 4 (source, gate, drain, body) |
| Channel types | 2 (n-channel NMOS, p-channel PMOS) |
| Example threshold voltage (nanowire MOSFET simulation) | ~0.45 V |
| Datasheet threshold test current | commonly 250 μA or 1 mA |
| Gate-oxide thickness spec, 90-nm process | 1 nm |
These figures come from TINA’s MOSFET chapter and Wikipedia’s threshold voltage article. They can change, so check the source before you rely on them.
How a MOSFET is built
The name spells out the layers: a metal oxide semiconductor stack that forms a field effect transistor. In an NMOS example from CircuitBread, a p-type substrate holds two heavily doped n-type regions, called the source and the drain.
An oxide that acts as an insulator is deposited on top of the substrate. A layer of metal on top of that finishes the gate.
According to TINA, the MOSFET is a four terminal device:
- Source (S): where carriers enter the channel.
- Gate (G): the insulated control terminal.
- Drain (D): where carriers leave the channel.
- Body or substrate: the fourth terminal, which is almost always connected to the source.
Because the gate is insulated from the channel, the gate current is negligibly small. That is why MOSFETs are sometimes called IGFETs, short for insulated gate field-effect transistors.
If transistors in general are new to you, the guide on how does a transistor work covers the wider family first.
How does the gate switch current on and off?
The gate has no direct electrical connection to the channel, yet it still controls it. As CircuitBread puts it, the voltage on the gate creates a field effect.
Here is the sequence for an n-channel enhancement-mode device:
- With no gate-to-source voltage, no conductive channel exists, and the device is off.
- A positive gate voltage attracts free electrons toward the surface under the oxide.
- Once enough electrons gather, they form an inversion layer that connects the source and the drain.
- With a voltage across the source and drain, current now flows, and more gate voltage widens the channel.
The minimum gate-to-source voltage that creates this conducting path is the threshold voltage. Below it, an enhancement-mode device conducts almost nothing.
The operating regions: cutoff, linear and saturation
A MOSFET behaves differently depending on its gate and drain voltages. CircuitBread stresses that an operating region is not a physical region, which can confuse beginners.
- Cutoff: the gate voltage is below the threshold, so the channel is not formed and almost no current flows.
- Linear (also called triode or ohmic): the gate is above the threshold, and the MOSFET behaves like a resistor whose resistance depends on the gate-to-source voltage.
- Saturation: at higher drain voltage the channel is constrained at the drain end, and further increases in drain-source voltage do not increase drain current.
Sources word these regions slightly differently, and the same region can carry several names. Check the definitions in your textbook or datasheet before you compare notes.
The Georgia Tech visualization describes a MOSFET as a voltage-controlled switch, and it lets you apply gate and drain voltages to see the pinch-off yourself.
NMOS, PMOS, enhancement and depletion types
Just like bipolar transistors, MOSFETs come in two polarities. TINA notes they can be either n-channel (NMOS) or p-channel (PMOS), and either enhancement or depletion mode.
- NMOS: a p-type substrate with electrons as carriers, and a positive threshold voltage.
- PMOS: an n-type substrate with holes as carriers, and a negative threshold voltage.
- Enhancement mode: the channel must be created by a gate voltage, and the n-channel version is the most commonly used form of field-effect transistor.
- Depletion mode: a physical channel already exists between drain and source, so current flows with zero gate voltage until a negative gate voltage pushes the electrons out.
Modern designs include nanometer-sized MOSFETs, so a natural next step is the guide on what is an integrated circuit.
Threshold voltage and what datasheets say
The threshold voltage is the minimum gate-to-source voltage that creates a conducting path between source and drain, per Wikipedia. The same page shows a nanowire MOSFET simulation where the channel connects at a ~0.45 V threshold.
Below the threshold, a small subthreshold leakage current still flows, and it varies exponentially with gate bias. For that reason, datasheets specify the threshold voltage at a measurable test current, commonly 250 μA or 1 mA.
Oxide thickness matters too. A thinner oxide lowers the threshold voltage but raises leakage, and the design specification for 90-nm gate-oxide thickness was set at 1 nm, according to the same Wikipedia article.
Wikipedia adds that in wide planar transistors the threshold is essentially independent of the drain-source voltage. It is less clear in modern nanometer-sized MOSFETs because of drain-induced barrier lowering.
Practical points for your first MOSFET circuit
- Do not stop at the threshold. When the gate-to-source voltage only just exceeds the threshold, very little current can flow, according to TINA.
- Mind the polarity. The threshold is a positive number for an n-channel device and a negative number for a p-channel device.
- Check the substrate. The source and drain must sit at equal or higher potential than the substrate, or you forward bias the PN junction and get unwanted current, as CircuitBread warns.
- Watch the body effect. When the body is not tied to the source, it shifts the threshold voltage.
Frequently asked questions
Is a MOSFET the same as a transistor?
Show answer
A MOSFET is one kind of transistor, specifically a field-effect transistor with an insulated gate. Other kinds, such as bipolar transistors, use different structures.
Why does the gate draw almost no current?
Show answer
The gate is separated from the channel by an insulating oxide layer. TINA states that the gate current is negligibly small for this reason.
Does the drain-source voltage change the threshold voltage?
Show answer
In wide planar devices it is essentially independent of drain-source voltage, according to Wikipedia. In modern nanometer-sized devices, drain-induced barrier lowering makes this less clear.
What does n+ mean in a MOSFET diagram?
Show answer
The source and drain are labeled n+ to indicate heavily doped n-regions, as the Wikipedia threshold voltage article explains.
